8 Channel Capacitive Touch Sensor with 2 LED Drivers
Datasheet
3.2.5
3.2.6
SMBus Stop Bit
The SMBus Stop bit is defined as a transition of the SMBus Data line from a logic ‘0’ state to a logic
‘1’ state while the SMBus clock line is in a logic ‘1’ state. When the CAP1128 detects an SMBus Stop
bit and it has been communicating with the SMBus protocol, it will reset its client interface and prepare
to receive further communications.
SMBus Timeout
The CAP1128 includes an SMBus timeout feature. Following a 30ms period of inactivity on the SMBus
where the SMCLK pin is held low, the device will timeout and reset the SMBus interface.
The timeout function defaults to disabled. It can be enabled by setting the TIMEOUT bit in the
Configuration register (see Section 5.6, "Configuration Registers").
2
3.2.7
SMBus and I C Compatibility
The major differences between SMBus and I2C devices are highlighted here. For more information,
refer to the SMBus 2.0 and I2C specifications. For information on using the CAP1128 in an I2C system,
refer to SMSC AN 14.0 SMSC Dedicated Slave Devices in I2C Systems.
1. CAP1128 supports I2C fast mode at 400kHz. This covers the SMBus max time of 100kHz.
2. Minimum frequency for SMBus communications is 10kHz.
3. The SMBus client protocol will reset if the clock is held at a logic ‘0’ for longer than 30ms. This
timeout functionality is disabled by default in the CAP1128 and can be enabled by writing to the
TIMEOUT bit. I2C does not have a timeout.
4. The SMBus client protocol will reset if both the clock and data lines are held at a logic ‘1’ for longer
than 200µs (idle condition). This function is disabled by default in the CAP1128 and can be enabled
by writing to the TIMEOUT bit. I2C does not have an idle condition.
5. I2C devices do not support the Alert Response Address functionality (which is optional for SMBus).
6. I2C devices support block read and write differently. I2C protocol allows for unlimited number of
bytes to be sent in either direction. The SMBus protocol requires that an additional data byte
indicating number of bytes to read / write is transmitted. The CAP1128 supports I2C formatting only.
3.3
SMBus Protocols
The CAP1128 is SMBus 2.0 compatible and supports Write Byte, Read Byte, Send Byte, and Receive
Byte as valid protocols as shown below.
All of the below protocols use the convention in Table 3.2.
Table 3.2 Protocol Format
DATA SENT
TO DEVICE
DATA SENT TO
THE HOST
Data sent
Data sent
SMSC CAP1128
Revision 1.32 (01-05-12)
DATA1S9HEET
SMSC [ SMSC CORPORATION ]
